We propose to study the process of health develpment during adolescence by conducting an experimental demonstration and scientific analysis of how intensive health promotion for youth can be accomplished. The research will include (a) longitudinal field experimentation to assess the effects of social psychological intervention on the onset of smoking and other behaviors deterimental to health, (b) longitudinal correlational study to model processes which underly the onset of those behaviors. In five site two cohorts of 150-300 6th or 7th graders will be identified. One of each of these matched pairs of cohorts will be randomly assigned to receive a four year program of systematic peer leadership designed to prevent smoking, misuse of alcohol and other drugs, and foolish physical risk taking and to promote alternatives to unhealthy behaviors. Both cohorts will be surveyed at base-line and over four years of follow-up to assess outcomes and analyze processes in health behavior development. Physiological measures of smoking (carbon monoxide in exhaled breath) and other objective data collection systems will be empolyed.